The White House says President Barack Obama and German Chancellor Angela Merkel have discussed potential further sanctions against Russia over its involvement in Ukraine.

The White House said in a statement that during a phone call on April 10, Obama and Merkel called again on Moscow to withdraw its troops from the border region.

The statement said that pro-Russian separatists, "apparently with support from Moscow," were destabilizing Ukraine through "an orchestrated campaign of incitement and sabotage."

It said Obama "underscored the need for the United States, European Union, and other global partners to be prepared to meet further Russian escalation with additional sanctions."

The White House said that the leaders will continue their talks during Merkel's forthcoming visit to Washington. No date has been announced.

The United States also accused Moscow of using energy to bully Ukraine after Russian President Vladimir Putin warned Europe that gas supplies to Ukraine could be suspended if Kyiv does not pay off its $2.2 billion gas debt.

“We condemn Russia’s efforts to use energy as a tool of coercion against Ukraine," U.S. State Department spokeswoman Jen Psaki said on April 10.

She noted that the price of $485 per 1,000 cubic meters Ukraine is now paying was "clearly not set by market forces and well above the average price paid by EU members.”

Psaki cited Putin’s letter earlier in the day to leaders of 18 European countries about the "critical situation" over Ukraine's debt, as well as Russia’s recent moves to hike gas prices sharply for Ukraine.

Putin's letter warned of a suspension of gas supplies to Ukraine if Kyiv does not pay off its $2.2 billion gas debt.

The Kremlin said on April 10 that Putin told the European leaders that Ukraine's debt could impact the transit of Russian gas to much of Europe.

He wrote that Gazprom would be "compelled to switch over to advance payment for gas deliveries" for Ukraine and that if Ukraine remains unable to settle its debt, Gazprom "will completely or partially cease gas deliveries."

Putin raised concerns about Ukraine siphoning off gas from pipelines leading to Europe and said Ukraine needed some 11.5 billion cubic meters of gas, worth some $5.5 billion, to fill the country's underground storage tanks.

Putin also wrote Russia is prepared to take part, along with the European Union, in efforts to restore Ukraine's economy.

But the Russian leader also said the EU's inaction regarding Ukraine's gas debt "gives one the impression that the European partners want to unilaterally blame Russia for the consequences of Ukraine's economic crisis."

At the same time, Putin's letter also makes reference to Russia "subsidizing Ukraine's economy" during the last four years at a cost of billions of dollars to Russian state coffers.

According to Russian news agency Interfax, leaders Putin's letter was sent to included the leaders of France, Germany, Austria, Italy, and Slovakia.

Meanwhile, in Washington, U.S. Assistant Secretary of State Victoria Nuland told an April 10 Senate hearing the Ukrainian crisis has also had an adverse effect on Russia's economy.

"More than $ 25 billion [were] spent by the Russian Federation over the last five to six weeks to prop up the ruble, to defend it," said Nuland.

She also mentioned the flight of capital from Russia in the first quarter of the year, which she said was "greater than capital flight throughout all of 2013."

Lew, speaking at the World Bank/IMF meetings in Washington, said the United States was ready to impose additional sanctions on Russia if it continued to step up tensions in Ukraine.

Lew's statement came as Russia's First Deputy Prime Minister Igor Shuvalov, speaking in Berlin on April 10, said Moscow has other potential partners if western countries turn elsewhere for oil and gas in response to the Ukrainian crisis.